Abstract. The predicted rise in anthropogenic CO₂ emissions will increase CO₂ concentrations and decrease seawater pH in the upper ocean. Recent studies have revealed effects of pCO₂ induced changes in seawater chemistry on a variety of marine life forms, in particular calcifying organisms. To test whether the predicted increase in pCO₂ will directly or indirectly (via changes in phytoplankton dynamics) affect abundance, activities, and community composition of heterotrophic bacteria during phytoplankton bloom development, we have aerated mesocosms with CO₂ to obtain triplicates with three different partial pressures of CO₂ (pCO₂): 350 μatm (1×CO₂), 700 μatm (2×CO₂) and 1050 μatm (3×CO₂). The development of a phytoplankton bloom was initiated by the addition of nitrate and phosphate. In accordance to an elevated carbon to nitrogen drawdown at increasing pCO₂, bacterial production (BPP) of free-living and attached bacteria as well as cell-specific BPP (csBPP) of attached bacteria were related to the C:N ratio of suspended matter. These relationships significantly differed among treatments. However, bacterial abundance and activities were not statistically different among treatments. Solely community structure of free-living bacteria changed with pCO₂ whereas that of attached bacteria seemed to be independent of pCO₂ but tightly coupled to phytoplankton bloom development. Our findings imply that changes in pCO₂, although reflected by changes in community structure of free-living bacteria, do not directly affect bacterial activity. Furthermore, bacterial activity and dynamics of heterotrophic bacteria, especially of attached bacteria, were tightly correlated to phytoplankton development and, hence, may also potentially depend on changes in pCO₂.